Technical Abstract:
Approximately half of the 67 species of Phytophthora are heterothallic; the rest are homothallic. If hetero- or homothallism dictates the mating system, there should be almost no heterozygosity in populations of homothallic Phytophthora species due to self fertilization. In contrast, heterothallic species should contain high levels of heterozygosity. However, levels of heterozygosity within species of Phytophthora so far have not been analyzed. To test whether there are differences in mating system, Wright's fixation inde (F = 1 - (H Obs / H Exp), where H Obs is the observed heterozygosity and H Exp the expected heterozygosity assuming random mating) was calculated for 16 species of Phytophthora by reanalysis of previously published data. As expected, fixation indices were near 1.0 for four of the six homothallic species. Fixation indices for four of the ten heterothallic species were between zero and 0.3, as expected for random mmating populations. The remaining species could be divided into three groups. One group, consisting of three hetero- and one homothallic species, probably had a mixed mating system with intermediate fixation index values near 0.5. A second group of heterothallic species had high fixation index values similar to those for homothallic species, probably due to asexual reproduction or inbreeding. A third group contained two heterothallic species with negative fixation index values. Deviations from expectation probably were due to asexual reproduction, incorrect scoring of some isozyme data, or possibly a Wahlund effect. Many Phytophthora species probably have a mixed mating system in nature that cannot be predicted on the basis of hetero- or homothallism. However, this conclusion must be confirmed by analyses of larger samples from carefully defined populations.